People say they beat level 6 if done correctly and have seen a guy on youtube do so but vid was sped up to a point to where I feel he may have had some cheats. I've done this on a hacked version and even then it is easy to let 20 creeps go by. I can get close to Wave 90 in normal with no hack but o matter what you better not lose one guy before then as to the flying waves at I think 92 and 98 or so are so hard even on the hacked version they were hard as hell.
Well ttips for getting past 72 aas best i can describe it is first make to paths across the top of the board and leaving the middle lower parts for freeze turrets and air missle turrets only leaving a small space between each. By the time you get to level 72 you should have around 7 fully upgraded air missle turrets and 2 fully upgraded freeze turrets. I place one freeze turrett right in the middle and one right before the top guys coming out. As far as beating it you just have to be really fast with selling,placing,selling and placing again with the two short paths at top. In the short paths should be about 3 plasma turrets and 2 ground missle turrets fully upgraded.